In this renewal application a paired antibody (Ab) tumor targeting system is being tested in which the first antibody is an anti-tumor Ab to which we have coupled a hapten, and the second Ab is an anti-hapten Ab that has been covalently linked to a cytotoxic drug, a toxin, a radioisotope or a modified lipid on the surface of liposomes containing a cytotoxic drug. By attaching multiple haptenic groups (up to 26 per Ab molecule) to the first antibody, it is possible to augment the binding of the second Ab-cytotoxic ligand complex. Other major advantages of this approach over conventional targeting protocols include: 1) the ability to purify antibodies on hapten affinity columns prior to conjugation to the cytotoxic agent or liposomes and to repurify the cytotoxic-Ab complexes after coupling, 2) an opportunity to select the second antibody from over one hundred will-characterized anti-hapten Abs that are currently available making it possible to determine the effects of Ab isotope, valence and affinity, 3) use of one set of second Abs with any other first antibody, and 4) an increase in the amount of both Ab1 and Ab2 internalized and a decrease in the amount shed (due to crosslinking of the first Ab bound to the human lung tumor with the second Ab). In order to pursue these studies, we have identified two tumor-associated molecules and have generated monoclonal Abs specific for these two tumor markers. The first molecule is a 160Kd glycoprotein (gp160) that is present on several different histological types of non-small cell human lung tumors but is not detectable in lung or most other normal tissues. The other tumor marker is the idiotype associated with a murine B-cell tumor. In both the human and murine tumor systems we have demonstrated that Abs are internalized following binding to the respective tumor markers. We have also established that the growth of these tumors in vitro can be selectively inhibited by radioimmunotherapy, immunotoxins or by immunospecific cytotoxic drug containing liposomes. Using conventional single Ab mediated targeting protocols in vivo, we have determined that immunospecific liposomes containing a cytotoxic drug suppress the spread of a primary tumor to another organ, but often fail to suppress the growth of the established primary tumor nodules. In the present proposal we expect to substantially improve upon current targeting efforts by using the paired antibody protocol, by increasing access to the tumor using photodynamic therapy and by eliminating host response to the targeting antibody by modulating the host's immune response.